U.S. patent application number 10/487956 was filed with the patent office on 2004-11-25 for conductive polymer having positive temperature coefficient, method of controlling positive temperature coefficient property of the same and electrical device using the same.
Invention is credited to Choi, Soo-An, Han, Joon-Koo, Kim, Do-Yun, Ko, Chang-Mo, Lee, Jong-Ho.
Application Number | 20040232387 10/487956 |
Document ID | / |
Family ID | 19713553 |
Filed Date | 2004-11-25 |
United States Patent
Application |
20040232387 |
Kind Code |
A1 |
Kim, Do-Yun ; et
al. |
November 25, 2004 |
Conductive polymer having positive temperature coefficient, method
of controlling positive temperature coefficient property of the
same and electrical device using the same
Abstract
PTC conductive polymer composition includes organic polymer
containing polyolefin components essentially consisting of
30.about.40 w % high density polyethylene (HDPE), 20.about.40 w %
low density polyethylene (LDPE) and 10.about.30 w %
ethylene-acrylic-acid (EAA) or ethylene-vinyl-acetate (EVA), and
20.about.30 w % high or low density polyethylene which is
denaturated into maleic anhydride compound; 60.about.120 w %
electrical conductive particles dispersed into the organic polymer,
the electrical conductive particles by weight of the organic
polymer; and 0.2.about.0.5 w % peroxidic cross-linking agent added
for cross-linking reaction by weight of the organic polymer. Thus,
it becomes possible to control PTC characteristics such as
switching temperature and trip time of an electrical device by
suitably adjusting an added amount of the polyethylene, which is
denaturated into maleic anhydride compound.
Inventors: |
Kim, Do-Yun; (Seoul, KR)
; Lee, Jong-Ho; (Kyunggi-do, KR) ; Choi,
Soo-An; (Kyunggi-do, KR) ; Han, Joon-Koo;
(Seoul, KR) ; Ko, Chang-Mo; (Kyunggi-do,
KR) |
Correspondence
Address: |
JONES DAY
222 EAST 41ST ST
NEW YORK
NY
10017
US
|
Family ID: |
19713553 |
Appl. No.: |
10/487956 |
Filed: |
February 25, 2004 |
PCT Filed: |
April 25, 2002 |
PCT NO: |
PCT/KR02/00762 |
Current U.S.
Class: |
252/500 |
Current CPC
Class: |
H01C 7/027 20130101;
H01C 17/06586 20130101 |
Class at
Publication: |
252/500 |
International
Class: |
H01B 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 25, 2001 |
KR |
2001/51568 |
Claims
What is claimed is:
1. An organic positive temperature coefficient (PTC) composite
which realizes PTC characteristics by dispersing electrical
conductive particles into organic polymer: wherein the conductive
composite includes 0.2.about.0.5 w % of peroxidic cross-linking
agent added into 100 w % of the organic polymer for cross-linking
reaction, and wherein the organic polymer comprises, (1) polyolefin
component containing 30.about.40 w % of high density polyethylene
(HDPE), 20.about.40 w % of low density polyethylene (LDPE) and
10.about.30 w % ethylene-acrylic-acid (EAA) or
ethylene-vinyl-acetate (EVA); and (2) 20.about.30 w % of HDPE or
LDPE, on which maleic anhydride is grafted, added to the polyolefin
component, whereby a switching temperature and a trip time are
controlled by suitably adjusting an added amount of the maleic
anhydride grafted polyethylene.
2. The organic PTC composite according to claim 1, wherein
60.about.120 w % of the electrical conductive particles are
dispersed into 100 w % of the organic polymer.
3. The organic PTC composite according to claim 2, further
comprising an antioxidant, which is 0.2 to 0.5% by weight of the
organic polymer.
4. The organic PTC composite according to claim 2, wherein the
organic PTC composite has a resistivity of 0.8.about.2.0 .OMEGA.-cm
at an ambient temperature.
5. The organic PTC composite according to claim 3, wherein the
organic PTC composite has a resistivity of 0.8.about.2.0 .OMEGA.-cm
at an ambient temperature.
6. A method of controlling positive temperature coefficient (PTC)
characteristics of an organic PTC composite which is made by
dispersing electrical conductive particles such as carbon black
into polyolefin component containing 30.about.40 w % of high
density polyethylene (HDPE), 20.about.40 w % of low density
polyethylene (LDPE) and 10.about.30 w % ethylene-acrylic-acid (EAA)
or ethylene-vinyl-acetate (EVA) and then cross-linking the
polyolefin component with peroxidic cross-linking agent, wherein
the method comprises the step of controlling a switching
temperature (Ts) and a trip time by adding 20.about.30 w % of HDPE
or LDPE, on which maleic anhydride is grafted, to the polyolefin
component.
7. The method of controlling PTC characteristics of the organic PTC
composite according to claim 6, wherein, as an added amount of the
maleic anhydride grafted polyethylene increases, the switching
temperature decreases and the trip time increases.
8. An electrical device comprising: 1) a PTC element including: a)
organic polymer made by adding 20.about.30 w % of high density
polyethylene (HDPE) or low density polyethylene (LDPE), on which
maleic anhydride is grafted, into polyolefin components containing
30.about.40 w % of HDPE, 20.about.40 w % of LDPE and 10.about.30 w
% ethylene-acrylic-acid (EAA) or ethylene-vinyl-acetate (EVA); b)
60.about.120 w % of electrical conductive particles dispersed into
100 w % of the organic polymer; and c) 0.2.about.0.5 w % of
peroxidic cross-linking agent added into 100 w % of the organic
polymer for cross-linking reaction, 2) a pair of electrodes
connectable to a power source, respectively, the electrodes
allowing current to flow through the PTC element when being
connected to the power source.
9. The electrical device according to claim 8, wherein, when
testing a current-time characteristic of the electrical device with
1,000 successive cyclic tests under the condition that the trip
time is set to a time when a resistance of the device becomes 10
.OMEGA. and an added overload current is set to 5A, a ratio R1/R0
is maintained between 1.0 and 1.5 at every test, where R1 is a
resistance after the test and R0 is a resistance before the
test.
10. The electrical device according to claim 9, wherein, in the
current-time characteristic test, the ratio R1/R0 is maintained
between 1.0 and 2.5 since the electrical device is in a tripped
state for 10 hours.
11. The electrical device according to claim 8, wherein, when
testing a temperature-resistance characteristic of the electrical
device with 10 successive cyclic tests, a ratio R2/R0 is maintained
between 1.0 and 2.0 at every test, where R2 is a resistance after
the test and R0 is a resistance before the test.
12. The electrical device according to claim 11, wherein the ratio
R2/R0 is maintained between 1.0 and 2.0 at every test even when a
ratio of a maximum resistance to a resistance at an ambient
temperature is more than 10.sup.6.
13. The electrical device as claimed in claim 12, wherein, in a
temperature-resistance test, a ratio R3/R0 is maintained more than
10.sup.5 at 140.degree. C. or more, where R3 is a peak resistance
and R0 is an initial resistance.
Description
TECHNICAL FIELD
[0001] The present invention relates to a positive temperature
coefficient (PTC) composite and an electrical device containing the
PTC composite. More particularly, the present invention relates to
a PTC composite, which is made by adding polyethylene, on which a
maleic anhydride is grafted, into a maleic anhydride for the
purpose of easy control of switching temperature and trip time.
BACKGROUND ART
[0002] PTC means a characteristic that electrical resistance
rapidly increases at a relatively narrow temperature range due to
increase of temperature. PTC composites have such PTC
characteristics and they are generally used in a circuit protection
element, which limits current of a circuit when the circuits such
as a heater, a positive-characterized thermistor, an ignition
sensor, a battery or the like are short-circuited. The circuit
protection element makes the circuit recovered when the cause of
the short circuit is removed.
[0003] As another example employing the PTC composites, there is a
PTC element in which at least two electrodes are electrically
connected to such composites. Such a PTC element is used as an
element for preventing over current or overheat, which acts for
self-control of temperature, as described above.
[0004] Over-current prevention mechanism using the PTC element is
as follows. At an ambient temperature, the PTC composite has a
sufficiently low resistance, so ensuring current flow through a
circuit. However, if a high current passes through the circuit due
to, for example, a short circuit, this high current causes Joule
heat generated in the PTC element, which increases temperature and
therefore raises resistance of the element by the PTC
characteristics. This resistance blocks current flow through the
element, so protecting the circuit. It is generally referred as a
current limiting property.
[0005] Such PTC element, or PTC composite, needs to have a current
limiting property, which can repeatedly work even under high
voltage. Also, improvement of the current limiting property comes
from sufficient decrease of an initial resistance of the PTC
element as well as endowment of the effective PTC
characteristics.
[0006] There are developed many kinds of PTC composites. As an
example, a PTC composite made by adding univalent or trivalent
metal oxide to BaTiO.sub.3 is already well known. However, such
composite has a problem that it allows current flow less than 1
msec because it shows NTC (Negative Temperature Coefficient)
characteristics right after the PTC characteristics is
manifested.
[0007] As an alternation, there has been developed a PTC composite,
which is made by dispersing electrical conductive particles such as
carbon black, carbon fiber, carbon graphite or metal particles to
an organic polymer such as polyethylene, polypropylene or
ethylene-acrylic acid copolymer. Such PTC composite is generally
made by blending a necessary amount of electrical conductive
particles into at least one resin, used as an organic polymer.
[0008] Reference can be made for example to U.S. Pat. No.
3,243,753, U.S. Pat. No. 3,823,217, U.S. Pat. No. 3,950,604, U.S.
Pat. No. 4,188,276, U.S. Pat. No. 4,272,471, U.S. Pat. No.
4,414,301, U.S. Pat. No. 4,425,397, U.S. Pat. No. 4,426,339, U.S.
Pat. No. 4,427,877, U.S. Pat. No. 4,429,216, U.S. Pat. No.
4,442,139 and so on.
[0009] In addition, Korean Patent Publication No. 99-63872
discloses a technique of grafting conductive particulate fillers
into maleic anhydride grafted polyethylene in order to make a PTC
composite. This PTC composite may show great adhesion to a metal
electrode with a soft surface, recover its initial or lower
resistance after repeated cycling (that is, changing from a low
resistance state to a high resistance state and then returning),
and extend a period of a tripped state.
[0010] However, any one among them does not show a technique to
control a switching temperature and a trip time by adding
polyethylene, on which a maleic anhydride is grafted, into
crystalline polymer compounds.
DISCLOSURE OF INVENTION
[0011] Inventors of the present invention have discovered that it
is possible to control a switching temperature and a trip time by
adding low-density polyethylene (LDPE) or high-density polyethylene
(HDPE), on which a maleic anhydride is grafted, into a mixture of
HDPE, LDPE, ethylene-ethyl acrylate copolymer (EEA),
ethylene-acrylic-acid (EAA) or ethylene-vinyl-acetate (EVA).
[0012] An object of the present invention is to provide a PTC
composite for easily controlling a switching temperature and a trip
time thereof, and a method of controlling such PTC
characteristics.
[0013] Another object of the present invention is to provide a PTC
composite with excellent heat-stability and conductivity by
conducting cross-linking reaction to conductive polymer compounds
with use of a peroxidic cross-linking agent.
[0014] In order to accomplish the above objects, the present
invention provides an organic positive temperature coefficient
(PTC) composite which includes organic polymer made by adding
20.about.30 w % of high density polyethylene (HDPE) or low density
polyethylene (LDPE) on which a maleic anhydride is grafted into
polyolefin components containing 30.about.40 w % of HDPE,
20.about.40 w % of LDPE and 10.about.30 w % ethylene-acrylic-acid
(EAA) or ethylene-vinyl-acetate (EVA); 60.about.120 w % of
electrical conductive particles dispersed into 100 w % of the
organic polymer; and 0.2.about.0.5 w % of peroxidic cross-linking
agent added into 100 w % of the organic polymer for cross-linking
reaction.
[0015] Thus, a switching temperature and a trip time can be
controlled by suitably adjusting an added amount of the maleic
anhydride grafted polyethylene.
[0016] As another aspect of the present invention, there is
provided a method of controlling positive temperature coefficient
(PTC) characteristics of an organic PTC composite which is made by
dispersing electrical conductive particles such as carbon black
into polyolefin component containing 30.about.40 w % of high
density polyethylene (HDPE), 20.about.40 w % of low density
polyethylene (LDPE) and 10.about.30 w % ethylene-acrylic-acid (EAA)
or ethylene-vinyl-acetate (EVA) and then cross-linking the
polyolefin component with peroxidic cross-linking agent, wherein
the method comprises the step of controlling a switching
temperature (Ts) and a trip time by adding 20.about.30 w % of HDPE
or LDPE on which a maleic anhydride is grafted to the polyolefin
component.
[0017] At this time, as an added amount of the maleic anhydride
grafted polyethylene increases, the switching temperature and the
trip time are also decrease.
[0018] As still another aspect of the present invention, there is
also provided an electrical device which includes a PTC element
having organic polymer made by adding 20.about.30 w % of high
density polyethylene (HDPE) or low density polyethylene (LDPE), on
which maleic anhydride is grafted into a maleic anhydride compound,
into polyolefin components containing 30.about.40 w % of HDPE,
20.about.40 w % of LDPE and 10.about.30 w % ethylene-acrylic-acid
(EAA) or ethylene-vinyl-acetate (EVA); 60.about.120 w % of
electrical conductive particles dispersed into 100 w % of the
organic polymer; and 0.2.about.0.5 w % of peroxidic cross-linking
agent added into 100 w % of the organic polymer for cross-lining
reaction, and a pair of electrodes connectable to a power source,
respectively, the electrodes allowing current to flow through the
PTC element when being connected to the power source.
[0019] Suggested in this invention is an organic PTC (Positive
Temperature Coefficient) composite which has a resistivity of
0.8.about.2.0 .OMEGA.-cm at an ambient temperature, shows excellent
temperature-resistance characteristic and current-time
characteristic and maintains its specific resistance to an initial
state after repeated increases and decreases of temperature.
[0020] More concretely, the organic PTC composite is made by adding
electrical conductive particulate fillers such as carbon block and
maleic anhydride grafted LDPE (or HDPE) into an organic polymer
compound containing HDPE, LDPE, EEA (Ethylene-ethyl Acrylate
Copolymer), EVA (Ethylene-Vinyl-Acetate), EAA
(Ethylene-Acrylic-Acid) and so on, and then cross-linking the
mixture with a cross-linking agent. The PTC composite may also
additionally include antioxidant, inert filler, stabilizer,
dispersing agent and so on.
[0021] The organic polymer of the present invention contains
30.about.40 w % of HDPE, 20.about.40 w % of LDPE and 10.about.30 w
% EAA, EVA or EEA.
[0022] A suitable content of maleic anhydride grafted HDPE or LDPE
added to the organic polymer is preferably 20.about.30 w %.
[0023] As the conductive particulate filler, powder nickel, gold
dust, powder copper, silvered powder copper, metal-alloy powder,
carbon black, carbon powder or carbon graphite can be used. Among
them, carbon black is most preferred as the conductive particulate
filler in the present invention.
[0024] An added amount of the carbon black is preferably about
30.about.60 w % by weight of the organic polymer.
[0025] An amount of the peroxidic cross-linking agent added for
cross-linking reaction is suitably about 0.3.about.0.8 w %.
[0026] In addition, a preferred amount of the antioxidant added as
an additional agent is 0.2.about.0.5 w %.
[0027] The organic PTC composite described above can be disposed
between two metal film electrodes to make an electrical device
having PTC characteristics. Such an electrical device having PTC
characteristics is described in FIG. 1. As shown in FIG. 1, the
electrical device includes two metal film electrodes 1 and a PTC
element 2 united between them. Such a PTC element 2 has the organic
PTC composite described above.
[0028] As the metal electrode, copper plating or nickel plating is
preferably used.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] These and other features, aspects, and advantages of the
present invention will become better understood with regard to the
following description, appended claims, and accompanying drawings,
in which like components are referred to by like reference
numerals. In the drawings:
[0030] FIG. 1 is a sectional view showing an electrical device
according to the present invention;
[0031] FIG. 2 is a graph for illustrating a temperature-resistance
characteristic of the composites according to first to fourth
embodiments of the present invention;
[0032] FIG. 3 is a graph for illustrating a temperature-resistance
characteristic of the composites according to second, fifth, sixth
and seventh embodiments of the present invention; and
[0033] FIG. 4 is a graph for illustrating a temperature-resistance
characteristic according to the second and fifth embodiments of the
present invention and a comparative example without using a
cross-linking agent.
BEST MODES FOR CARRYING OUT THE INVENTION
[0034] Hereinafter, a PTC composite and a method of making an
electrical device using the PTC composite according to the present
invention will be described in detail.
[0035] A mixture including organic polymer made by adding
20.about.30 w % of high density polyethylene (HDPE) or low density
polyethylene (LDPE) on which maleic anhydride is grafted into
polyolefin components containing 30.about.40 w % of HDPE,
20.about.40 w % of LDPE and 10.about.30 w % ethylene-acrylic-acid
(EAA) or ethylene-vinyl-acetate (EVA); 60.about.120 w % of
electrical conductive particles dispersed into 100 w % of the
organic polymer; and 0.2.about.0.5 w % of peroxidic cross-linking
agent added into 100 w % of the organic polymer for cross-linking
reaction is blended in a Banbury mixer during 20.about.30 minutes
at above a melting temperature.
[0036] The blended mixture is molded at a temperature of
140.degree. C. for 2 minutes under a pressure of 300 kg/cm.sup.2 to
make a PTC element of 5 mm thickness.
[0037] This PTC element is bonded to the metal electrodes at a
suitable temperature, and then cross-linked and cooled to
eventually make the electrical device as shown in FIG. 1.
[0038] The electrical device has the PTC element (or, conductive
complex) surrounded by two metal film electrodes, in which the
metal electrodes has a thickness of 15.about.50 .mu.m and the PTC
element has a thickness of 150.about.400 .mu.m. The finished
electrical device has a disk shape, and more preferably, has a
doughnut shape with a suitable-sized hole at its center.
[0039] Now, embodiments of the present invention are described in
detail.
Embodiment 1
[0040] Make an organic PTC composite by adding 70 w % of carbon
black, 0.3 w % of antioxidant and 0.2 w % of peroxidic
cross-linking agent into 100 w % of the organic polymer which
contains 35 w % of HDPE (High-Density. Polyethylene) having a
density of 0.95.about.0.965 g/cm.sup.3 and a 3.about.6 melt index,
35 w % of LDPE (Low-Density Polyethylene) having a density of
0.90.about.0.93 g/cm.sup.3 and a 3.about.6 melt index and 30 w % of
EVA (Ethylene-Vinyl Acetate).
Embodiment 2
[0041] Make an organic PTC composite by adding 70 w % of carbon
black, 0.3 w % of antioxidant and 0.2 w % of peroxidic
cross-linking agent into 100 w % of the organic polymer which
contains 30 w % of HDPE having a density of 0.95.about.0.965
g/cm.sup.3 and a 3.about.6 melt index, 30 w % of LDPE having a
density of 0.90.about.0.93 g/cm.sup.3 and a 3.about.6 melt index,
10 w % of EVA and 30 w % of LDPE on which maleic anhydride is
grafted having a density of 0.90.about.0.93 g/cm.sup.3 and a
3.about.6 melt index.
Embodiment 3
[0042] Make an organic PTC composite by adding 70 w % of carbon
black, 0.3 w % of antioxidant and 0.2 w % of peroxidic
cross-linking agent into 100 w % of the organic polymer which
contains 35 w % of HDPE having a density of 0.95.about.0.965
g/cm.sup.3 and a 3.about.6 melt index, 35 w % of LDPE having a
density of 0.90.about.0.93 g/cm.sup.3 and a 3.about.6 melt index,
10 w % of EVA and 20 w % of LDPE on which maleic anhydride is
grafted having a density of 0.90.about.0.93 g/cm.sup.3 and a
3.about.6 melt index.
Embodiment 4
[0043] Make an organic PTC composite by adding 70 w % of carbon
black, 0.3 w % of antioxidant and 0.2 w % of peroxidic
cross-linking agent into 100 w % of the organic polymer which
contains 40 w % of HDPE having a density of 0.95.about.0.965
g/cm.sup.3 and a 3.about.6 melt index, 40 w % of LDPE having a
density of 0.90.about.0.93 g/cm.sup.3 and a 3.about.6 melt index,
10 w % of EVA and 10 w % of LDPE on which maleic anhydride is
grafted having a density of 0.90.about.0.93 g/cm.sup.3 and a
3.about.6 melt index.
Embodiment 5
[0044] Make an organic PTC composite by adding 70 w % of carbon
black, 0.3 w % of antioxidant and 0.2 w % of peroxidic
cross-linking agent into 100 w % of the organic polymer which
contains 30 w % of HDPE having a density of 0.95.about.0.965
g/cm.sup.3 and a 3.about.6 melt index, 30 w % of LDPE having a
density of 0.90.about.0.93 g/cm.sup.3 and a 3.about.6 melt index,
10 w % of EVA and 30 w % of HDPE on which maleic anhydride is
grafted having a density of 0.95.about.0.965 g/cm.sup.3 and a
3.about.6 melt index.
Embodiment 6
[0045] Make an organic PTC composite by adding 70 w % of carbon
black, 0.3 w % of antioxidant and 0.2 w % of peroxidic
cross-linking agent into 100 w % of LDPE on which maleic anhydride
is grafted having a density of 0.90.about.0.93 g/cm.sup.3 and a
3.about.6 melt index.
Embodiment 7
[0046] Make an organic PTC composite by adding 70 w % of carbon
black, 0.3 w % of antioxidant and 0.2 w % of peroxidic
cross-linking agent into 100 w % of HDPE on which maleic anhydride
is grafted having a density of 0.95.about.0.965 g/cm.sup.3 and a
3.about.6 melt index.
COMPARATIVE EXAMPLE 1
[0047] Do not add the peroxidic cross-linking agent to the organic
polymer of the second embodiment, so making a PTC composite without
cross-linking reaction.
COMPARATIVE EXAMPLE 2
[0048] Do not add the peroxidic cross-linking agent to the organic
polymer of the fifth embodiment so making a PTC composite without
cross-linking reaction.
[0049] Hereinafter, tests for temperature-resistance
characteristics and current-time characteristics of the PTC
composite in each embodiment and each comparative example are
presented.
Test 1
[0050] A test method and experimental instruments used for testing
the temperature-resistance characteristics are as follows.
[0051] 1) Test Sample
[0052] The sample for the test 1 is obtained by uniting the PTC
composites of the embodiments 1 to 4 with the metal electrodes,
cross-linking the united device with pressure for 20.about.30
minutes and then cooling it for 10 minutes.
[0053] 2) Test Method
[0054] a temperature range for measurement: -40.degree.
C..about.180.degree. C.
[0055] a temperature interval for measurement: 10.degree. C.
[0056] a waiting period at each measurement temperature: 15
minutes
[0057] 3) Experimental Instruments
[0058] a temperature rising/falling rate in a chamber: at least
1.degree. C./min
[0059] a resistance measuring device: HP 34401A (test current: less
than 1 mA, measuring range: 0.1 m.OMEGA..about.100 M.OMEGA.)
[0060] Results of the test 1 for the temperature-resistance
characteristics of the test sample according to the embodiments of
the present invention are well shown in FIG. 2.
[0061] As shown in FIG. 2, it can be easily understood that a
switching temperature of the PTC composite increases as an added
amount of the polyolefin, on which maleic anhydride is grafted,
decreases. In other words, it can be easily found that a switching
temperature of the embodiment 4 is greater than that of the
embodiment 2. At this time, the switching temperature means a
temperature at the point that a resistance suddenly increases
depending on changing temperature. Therefore, it should be
acknowledged that the switching temperature could be determined as
desired by adjusting an added amount of the polyolefin on which
maleic anhydride is grafted.
[0062] In addition, a resistance after repeated measurements of the
temperature-resistance characteristics (R2) and a resistance before
the measurement (R0) are compared. The electrical device of the
present invention maintains a ratio R2/R0 less than 2.0 at every
test until 1,000 times of the test, and preferably
1.0.about.2.0.
[0063] Moreover, the electrical device also maintains the ratio
R2/R0 between 1.0 and 2.0 even when a ratio of a maximum resistance
to a resistance at an ambient temperature is more than
10.sup.6.
Test 2
[0064] A test method and experimental instruments used for testing
the current-time characteristics are as follows.
[0065] 1) Test Sample
[0066] The test sample for the test 2 is obtained by uniting the
PTC composites of the embodiments 1 to 7 with the metal electrodes,
cross-linking the united device with pressure for 20.about.30
minutes and then cooling it for 10 minutes.
[0067] 2) Test Method
[0068] a set voltage: 15V DC (depending on conditions)
[0069] a set current: 10A DC (depending on conditions)
[0070] a time interval for measurement: 10 ms
[0071] 3) Experimental Instruments
[0072] a power supply: 20V/40A DC
[0073] a voltage and current measuring device: shunt (1.01V/0.01A
resolution) was used
[0074] 4) Trip Time
[0075] The trip time is defined as the time taken for a fault
current to be reduced as much as 1/2. For example, if voltage and
current are set as 15V/10A, the trip time is a time required to
decrease the current to 5A. At this time, the resistance of the PTC
element becomes 3 .OMEGA..
[0076] Results of the test 2 for the current-time characteristics
of the test sample according to the embodiments of the present
invention are described in Table 1 below.
1 TABLE 1 Embodiment 1 2 3 4 5 6 7 Trip time 4.about.5 7.about.8
6.about.7 5.about.6 7.about.8 8.about.9 9.about.10 (sec)
[0077] As shown in Table 1, it can be easily understood that a trip
time of the PTC composite decreases as an added amount of the
polyolefin on which maleic anhydride is grafted decreases. In
particular, the trip time decreases as an added amount of LDPE on
which maleic anhydride is grafted decreases. However, if the PTC
composite consists of only polyethylene on which maleic anhydride
is grafted like the embodiments 6 and 7, the trip time rather tends
to increase.
[0078] In addition, a resistance after repeated measurements of the
temperature-resistance characteristics (R1) and a resistance before
the measurement (R0) are compared. The electrical device of the
present invention maintains a ratio R1/R0 less than 1.5 at every
test until 1,000 times of the test, and preferably between 1.0 and
1.5.
[0079] Moreover, in test for a current-time characteristics, the
electrical device also maintains the ratio R1/R0 between 1.0 and
2.5 after 10 hours in a tripped state.
Test 3
[0080] Temperature-resistance characteristics for an electrical
device containing the PTC composites of the embodiments 2 and 5 and
an electrical devices containing PTC composites of the comparative
examples 1 and 2 which is made without cross-linking reaction are
tested with the same method as the test 1.
[0081] Results of the test 3 are well shown in FIG. 4. As shown in
FIG. 4, the electrical devices according to the embodiments 2 and 5
experiencing cross-linking reaction maintain a resistance more than
1,000 .OMEGA. at above 140.degree. C., while the electrical devices
of the comparative examples have a resistance less than 1,000
.OMEGA. at above 140.degree. C.
[0082] In other words, supposing that a resistance of an electrical
device at more than 140.degree. C. is R3 and an initial resistance
at an ambient temperature is R0, the electrical devices of the
embodiments 2 and 5 maintain a ratio R3/R0 more than 10.sup.5,
while the electrical devices of the comparative examples shows the
ratio R3/R0 less than 10.sup.5.
INDUSTRIAL APPLICABILITY
[0083] Therefore, the electrical device using the organic PTC
composite of the present invention has an advantage that its PTC
characteristics can be controlled as desired by adjusting an added
amount of polyethylene on which maleic anhydride is grafted into
maleic anhydride.
[0084] In particular, as an added amount of the maleic anhydride
grafted polyethylene decreases, the switching temperature increases
and the trip time decreases.
[0085] In addition, the electrical device of the present invention,
which is made using chemical cross-linking reaction with peroxidic
cross-linking agent, shows excellent heat stability rather than
other electrical devices, which have not experienced the
cross-linking reaction.
[0086] The organic PTC composite, the method of controlling the PTC
composite and the electrical device containing the PTC composite
according to the present invention have been described in detail.
However, it should be understood that the detailed description and
specific examples, while indicating preferred embodiments of the
invention, are given by way of illustration only, since various
changes and modifications within the spirit and scope of the
invention will become apparent to those skilled in the art from
this detailed description.
* * * * *